The Effect of Material Strain Hardening on the Determination of CTOD R-Curves Using SENB Specimens

In ductile materials the fracture toughness is usually characterised by a tearing resistance curve, or R-curve, plotting the fracture toughness in terms of J or CTOD against crack extension. Recent research has evaluated the methods to determine CTOD in engineering alloys with a wide range of yield to tensile (Y/T) ratios for single point CTOD. This work develops the investigation into R-curves, and reviews the assumptions about SENB specimens deforming under rigid rotation, the evaluation of CTOD from J for R-curves, and the nature of tearing initiation in low Y/T ratio stainless steel, from comparisons against a series of silicone replicas cast from the SENB specimen notch during fracture toughness tests. For CTOD R-curves, the methods based on CTOD from J in ISO 12135 and ASTM E1820 gave lower and less accurate R-curves than the rigid rotation methods in BS 7448-4 and WES 1108. However, the accuracy of the BS 7448-4 formula varied for the different strain hardening materials, and overestimated the R-curves in the low tensile ratio stainless steel. Investigations into the effect of the assumption about rigid rotation in different strain hardening materials led to a rotational factor function of tensile ratio, rp sh, to be developed from numerical modelling. When this function was substituted into standard equations in place of the fixed value of rp an improvement in the accuracy of BS 7448-4 R-curves compared to replica measurements was seen for the range of strain hardening investigated, but it did not significantly improve the accuracy of the WES 1108 formula, which accounts for strain hardening in other parameters. A combination of the elastic CTOD part of the WES 1108 formula, with the plastic CTOD incorporating the modified rotational factor, was concluded to offer the optimum method to determine CTOD R-curves for a range of strain hardening materials.

Plastic Rotational Factor Calculation for Shallow-Notched SE(B) Specimens

The plastic part of crack tip opening displacement (CTOD) is derived from the plastic hinge model for deep-notched single edge-notch bend (SE(B)) specimens in BS, WES and ISO CTOD testing standards, and a typical plastic rotational factor is given by a constant value of 0.4. This value is appropriate for deep-notched SE(B) specimens, but is not suitable for shallow-notched SE(B) specimens. In this study, a new equation of calculating the plastic rotational factor was obtained by using the Electric Power Research Institute (EPRI) scheme. The equation shows the effect of crack length and strain hardening on the plastic rotational factor, and is useful for evaluating CTOD in shallow-notched SE(B) specimens.